In a cellular wireless communication network typified by mobile phones, a wide service area is configured by decentrally deploying a plurality of radio base stations. Each radio base station forms a radio cell that is a range within which communication can be performed with the radio base station. Usually, approximately one to six radio cells are managed by one radio base station. In addition, coverage of each of mutually adjacent radio cells is partially overlapped with each other, and thereby communication can be maintained by a handover also in a radio terminal (UE: User Equipment) moving across the radio cells.
At the time of installation of the radio base station and during operation thereof, coverage optimization of the radio cell is performed for the purposes of reducing an area where communication cannot be performed (coverage hole) and of improving communication quality of the UE, particularly a throughput thereof. Generally, a drive test using a specialized measuring instrument is carried out on the field. In the drive test, a received power and an interference condition of a radio wave, abnormal disconnection of communication and occurrence of handover failure, a throughput, etc. are measured. A location where the received power is insufficient (weak coverage), a location where a strong interference has been received (pilot pollution), etc. are then identified, and adjustment of a radio parameter to solve these problems is performed. As the radio parameter to be adjusted, for example, an antenna tilt angle, an antenna azimuth, a transmission power, a handover parameter, etc of a radio cell are generally used.
Since the coverage optimization of the radio cell based on the above-mentioned drive test involves manual measurement and tuning work of the radio parameter, the operational cost of the wireless communication network increases. Consequently, in order to reduce the cost for such coverage optimization of the radio cell, a technology to autonomously optimize the coverage of the radio cell has been proposed.
According to a technique disclosed in Patent Literature 1, a femto cell (A) receives a measurement result of radio quality from a UE connected to the femto cell (A). When the radio quality does not satisfy an acceptable value, the femto cell (A) then instructs an adjacent femto cell (B) to reduce a transmission power. The femto cell (B) instructed to reduce the transmission power predicts radio quality after reduction of a transmission power of the UE connected to the femto cell (B), for example, an SINR (Signal to Interference plus Noise Ratio). When the predicted SINR satisfies the acceptable value, the femto cell (B) then carries out reduction of the transmission power as instructed by the femto cell (A).
According to a technique disclosed in Patent Literature 2, a radio cell measures its (own cell's) traffic load. At the time of a high traffic load, the radio cell then selects a radio cell which largely overlaps with the own cell from neighboring cells with a low traffic load. Coverage of the selected radio cell is then expanded, and coverage of the own cell is narrowed. Meanwhile, when the own cell's traffic load is low, coverage of a neighboring cell is narrowed, and coverage of the own cell is expanded.